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United States Patent |
5,209,944
|
Opower
,   et al.
|
May 11, 1993
|
Process for coating a substrate using a pulsed laser beam
Abstract
In order to so improve a process for the manufacture of a layer on a
substrate wherein by means of a pulsed laser beam a coating material is
ablated from a carrier, transported in the form of a current of particles
to the substrate and deposited on the latter to form the layer, that it is
suitable for the manufacture of thin precision layers, preferably in a
defined number of atom layers, as is, for example, required in the
manufacture of semiconductors, it is proposed that the layer be
manufactured as thin precision layer by pulse duration and pulse intensity
of the laser beam being selected such that ionization and heating-up of
the coating material take place essentially only in the zone of
interaction of the laser light with the coating material so the stream of
particles is formed as cluster-free plasma containing single, non-coherent
ions or atoms.
Inventors:
|
Opower; Hans (Krailling, DE);
Klose; Manfred (Stuttgart, DE)
|
Assignee:
|
Deutsche Forschungsanstalt fuer Luft- und Raumfahrt e.V. (Bonn, DE)
|
Appl. No.:
|
730550 |
Filed:
|
July 16, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
427/569; 219/121.62; 219/121.85; 427/582; 427/596 |
Intern'l Class: |
B05D 005/12; B05D 003/06; B23K 026/04; B23K 026/00 |
Field of Search: |
427/53.1,42
505/731,732
219/121.62,121.85
|
References Cited
U.S. Patent Documents
4281030 | Jul., 1981 | Silfvast | 427/53.
|
4701592 | Oct., 1987 | Cheung | 427/53.
|
4987007 | Jan., 1991 | Wagal et al. | 427/53.
|
5015492 | May., 1991 | Venkatesan et al. | 427/53.
|
5019552 | May., 1991 | Balooch et al. | 427/53.
|
5049405 | Sep., 1991 | Cheung | 427/53.
|
5049406 | Sep., 1991 | Geittner et al. | 427/53.
|
5080753 | Jan., 1992 | Doll et al. | 427/53.
|
5084300 | Jan., 1992 | Zander et al. | 427/53.
|
5094880 | Mar., 1992 | Hongoh | 427/53.
|
5097793 | Mar., 1992 | Shuhara et al. | 427/42.
|
5126165 | Jun., 1992 | Akihama et al. | 427/53.
|
5145713 | Sep., 1992 | Venkatesan et al. | 427/53.
|
Foreign Patent Documents |
263306 | Dec., 1988 | DD.
| |
Primary Examiner: Padgett; Marianne
Attorney, Agent or Firm: Lipsitz; Barry R.
Claims
What is claimed is:
1. A process for the manufacture of a layer on a substrate, wherein by
means of a pulsed laser beam a coating material is ablated from a source,
transported in the form of a stream of particles to the substrate and
deposited on the latter to form the layer, and wherein said layer is
produced by selecting a pulse duration and pulse intensity for said laser
beam such that ionization and heating-up of said coating material take
place essentially only in a zone of interaction of light from the laser
beam with said coating material so that said stream of particles is formed
as cluster-free plasma containing single, non-coherent ions or atoms.
2. A process as defined in claim 1, wherein an ablated layer of said
coating material has a thickness that corresponds at most to the primary
absorption length of said laser beam in the solid state of said coating
material.
3. A process as defined in claim 1, wherein a layer of said coating
material on said source with a thickness on the order of magnitude of one
hundred or several hundred atom layers is ionized and heated up.
4. A process as defined in claim 1, wherein the pulse duration and the
pulse intensity of said laser beam are selected such that a layer of said
coating material on said source with a thickness of approximately 100 to
1000 angstrom is ionized and heated up.
5. A process as defined in claim 1, wherein said laser beam with an angular
frequency .omega..sub.L corresponding to its wavelength strikes a surface
of said source with an intensity I.sub.L which is greater than
approximately
##EQU22##
I.sub.O being approximately equal to
##EQU23##
with N.sub.F as particle density in the solid state, E.sub.i as mean
kinetic energy per ion with ionization energy being neglected or mean
kinetic energy per atom in the stream of particles, M as mean mass per ion
or atom in the stream of particles and .omega..sub.O as plasma frequency
of a plasma with a density on the order of magnitude of the solid state
density N.sub.F.
6. A process as defined in claim 1, wherein said laser beam with an angular
frequency .omega..sub.L corresponding to its wavelength strikes a surface
of said source with an intensity I.sub.L which is smaller than
approximately
##EQU24##
I.sub.O being approximately equal to
##EQU25##
with N.sub.F as particle density in the solid state, E.sub.i as mean
kinetic energy per ion with ionization energy being neglected or mean
kinetic energy per atom in the stream of particles, M as mean mass per ion
or atom in the stream of particles and .omega..sub.O as plasma frequency
of a plasma with a density on the order of magnitude of the solid state
density N.sub.F.
7. A process as defined in claim 5, wherein said laser beam strikes the
surface of said source with an intensity I.sub.L which lies in the range
of between approximately
##EQU26##
and approximately
##EQU27##
8. A process as defined in claim 7, wherein said laser beam strikes the
surface of said source with an intensity I.sub.L which lies in the range
of between approximately
##EQU28##
and approximately
##EQU29##
9. A process as defined in claim 1, wherein said laser beam has an angular
frequency .omega..sub.L corresponding to its wavelength which during at
least a part of the pulse duration is greater than or equal to a plasma
frequency .omega..sub.P, with the plasma frequency .omega..sub.P being
determinable at any point in time from
##EQU30##
d being a thickness of a layer of coating material on said source
initially covered by the laser pulse, t a time from a start of the pulse
onwards and v.sub.i a velocity of the expansion of the plasma.
10. A process as defined in claim 9, wherein said laser beam angular
frequency .omega..sub.L, during at least approximately one third of the
pulse duration .tau., is greater than or equal to the plasma frequency
.omega..sub.P.
11. A process as defined in claim 10, wherein said laser beam angular
frequency .omega..sub.L, during at least approximately half of the pulse
duration .tau., is greater than or equal to the plasma frequency
.omega..sub.P.
12. A process as defined in claim 1, wherein said laser beam with an
angular frequency .omega..sub.L corresponding to its wavelength has a
pulse duration .tau. which is greater than approximately
##EQU31##
wherein
.tau..sub.O .apprxeq.d.times.E.sub.i.sup.-1/2 .times.M.sup.1/2
with d being a thickness of a layer of coating material on said source
material initially covered by the laser pulse, E as mean kinetic energy
per ion with the ionization energy being neglected or mean kinetic energy
per atom in the stream of particles, and M as mean mass per ion or atom in
the stream of particles.
13. A process as defined in claim 1, wherein said laser beam with an
angular frequency .omega..sub.L corresponding to its wavelength has a
pulse duration .tau. which is less than approximately
##EQU32##
wherein
.tau..sub.O .apprxeq.d.times.E.sub.i.sup.-1/2 .times.M.sup.1/2
with d being a thickness of the layer of coating material on said source
initially covered by the laser pulse, E.sub.i as mean kinetic energy per
ion with the ionization energy being neglected or mean kinetic energy per
atom in the stream of particles, and M as mean mass per ion or atom in the
stream of particles.
14. A process as defined in claim 12, wherein said laser beam pulse
duration .tau. lies in the range of between approximately
##EQU33##
and approximately
##EQU34##
15. A process as defined in claim 13, wherein said laser beam pulse
duration .tau. lies in the range of between approximately
##EQU35##
and approximately
##EQU36##
16. A process as defined in claim 1, wherein the pulse duration .tau. is
selected so short that any effect of an expansion of the plasma is
negligible.
17. A process as defined in claim 1, wherein a pulse intensity I.sub.L and
pulse duration .tau. of said laser beam are selected such that
I.sub.L .times..tau..apprxeq.H
with H lying between approximately 1 and 50 watt-seconds/cm.sup.2.
18. A process as defined in claim 1, wherein the laser beam has a
wavelength of substantially 0.2 .mu.m and a pulse duration in the range of
between approximately 1 and approximately 10 psec.
19. A process as defined in claim 1, wherein said laser beam has a
wavelength of smaller than 0.3 .mu.m.
20. A process as defined in claim 1, wherein a stream of particles with
ions or atoms of a mean kinetic energy E.sub.i on the order of magnitude
of approximately 10 eV to approximately 100 eV is generated.
21. A process as defined in claim 1, wherein an Nd solid-state laser is
used to provide said laser beam.
22. A process as defined in claim 1, wherein a Ti-sapphire laser is used to
provide said laser beam.
23. A process as defined in claim 1, wherein an excimer laser is used to
provide said laser beam.
24. A process as defined in claim 12, wherein the laser beam having pulse
duration .tau. is generated using mode-coupled laser radiation.
25. A process as defined in claim 9, wherein the wavelength of said laser
beam is provided by frequency multiplication of a fundamental wavelength.
Description
The invention relates to a process for the manufacture of a layer on a
substrate wherein by means of a pulsed laser beam a coating material is
ablated from a carrier, transported in the form of a stream of particles
to the substrate and deposited on the latter to form the layer.
Such a process is generally known for the coating of a substrate with laser
ablation, and this process is usually employed for applying macroscopic
coatings to the substrate.
The object underlying the invention is to so improve this known process
that it is suited for the manufacture of thin precision layers, preferably
in a defined number of atom layers, as is, for example, required in the
manufacture of semiconductors.
This object is accomplished in accordance with the invention in a process
of the kind described at the beginning by the layer being produced as a
thin precision layer by pulse duration and the pulse intensity of the
laser beam being selected such that ionization and heating-up of the
coating material take place essentially only in the zone of interaction of
the laser light with the coating material so the stream of particles is
formed as cluster-free plasma containing single, non-coherent ions or
atoms.
Hence the gist of the invention resides in controlling the creation of the
stream of particles so as to prevent a considerably larger area than the
zone of interaction of the laser light with the coating material from
being ablated by heat conduction processes, which would result in the
danger of the stream of particles including clusters, i.e., coherent atoms
or ions. The inventive selection of pulse duration and pulse intensity of
the laser beam such that the coating material will only be ablated in the
zone of interaction of the laser light with the coating material ensures
that the stream of particles will be substantially free of clusters and
coherent atoms or ions. Hence, all disturbances in a thin precision layer
which are caused by clusters or coherent ions and--for example, in
semiconductor technology--would lead to the precision layer being unfit
for use at least in the area of the disturbance, will be eliminated.
It is particularly expedient for the thickness of an ablated layer of the
coating material to correspond at the most to the primary absorption
length of the laser beam in the solid state of the coating material, so
only those atoms of the coating material that interact directly with the
laser beam are ablated.
It is particularly favorable for the coating material to be ionized and
heated up with a thickness of the order of magnitude of one hundred or
several hundred atom layers.
In a particularly expedient embodiment, provision is made for the pulse
duration and pulse length to be selected such that a layer of the coating
material is ionized and heated up with a thickness of approximately one
hundred to one thousand angstroms.
In the embodiments of the inventive process described so far, no general
limitation was given for the intensity, in particular in relation to the
other individual parameters of the inventive process. For the inventive
process to work particularly advantageously, the laser beam with an
angular frequency .omega..sub.L corresponding to its wavelength should
strike the surface of the coating material with an intensity I.sub.L which
is greater than approximately
##EQU1##
the intensity I.sub.O being approximately
##EQU2##
with N.sub.F as particle density in the solid state, E.sub.i as mean
energy per ion or atom in the stream of particles, M as mass of the ions
or atoms and .omega..sub.O as plasma frequency of a plasma with a density
of the order of magnitude of the density of the solid state of the coating
material.
In the inventive process, it is, furthermore, advantageous for the laser
beam with an angular frequency .omega..sub.L corresponding to its
wavelength to strike the surface of the coating material with an intensity
I.sub.L which is less than approximately
##EQU3##
In an advantageous choice of parameters for the inventive process, the
laser beam strikes the surface of the coating material with an intensity
I.sub.L which lies in the range of between approximately
##EQU4##
and approximately
##EQU5##
With intensities within the indicated range it is ensured that the stream
of particles will be substantially free of clusters and coherent atoms and
ions.
In a range which is particularly advantageous, the laser beam has an
intensity I.sub.L lying within the range of between approximately
##EQU6##
and approximately
##EQU7##
Optimum results are achieved with this set of parameters.
In all of the embodiments described so far, no special details were given
about the pulse duration in conjunction with the wavelength of the laser
beam. It is particularly advantageous to carry out the process with the
laser beam having an angular frequency .omega..sub.L corresponding to its
wavelength which is greater than or equal to the plasma frequency
.omega..sub.P during a considerable part of the pulse duration .tau., with
the development of the plasma frequency .omega..sub.P with respect to time
resulting from the approximate expansion equation
##EQU8##
with d being the thickness of the layer of coating material initially
covered by the laser beam, t the time from the start of the pulse onwards
and v.sub.i the velocity of the expansion of the plasma.
This condition advantageously results in penetration of the laser beam into
the plasma at least during a considerable part of the pulse duration and
hence in optimum heating-up of all ions or atoms of the plasma.
It is particularly advantageous for the laser beam to have an angular
frequency .omega..sub.L which during at least approximately a third of the
pulse duration .tau. is greater than or equal to the plasma frequency
.omega..sub.P.
It is even more advantageous for the laser beam to have an angular
frequency .omega..sub.L which during at least approximately half of the
pulse duration .tau. is greater than or equal to the plasma frequency
.omega..sub.P.
In the explanation of the above embodiments and conditions for the
intensity and the wavelength, no details of an advantageous connection
between the pulse duration and the wavelength were given. Within the scope
of an embodiment of the inventive solution, it has proven particularly
advantageous for the laser beam with an angular frequency .omega..sub.L
corresponding to its wavelength to have a pulse duration .tau. which is
greater than approximately
##EQU9##
wherein approximately
##EQU10##
with d as thickness of the ablated layer of coating material, E.sub.i as
mean energy per ion or atom in the stream of particles and M as mass per
ion or atom in the stream of particles.
A further advantageous marginal condition results from the laser beam with
an angular frequency .omega..sub.L corresponding to its wavelength having
a pulse duration .tau. which is smaller than approximately
##EQU11##
with .tau..sub.O being determinable as indicated hereinabove.
In a particularly preferred range for the pulse duration .tau., the latter
lies in the range of between approximately
##EQU12##
and approximately
##EQU13##
It is particularly preferred for the process to be carried out with the
pulse duration .tau. lying in the range of between approximately
##EQU14##
and approximately
##EQU15##
In accordance with the invention, it is, furthermore, particularly
advantageous for the pulse duration .tau. to be selected so short that the
effect of the expansion of the plasma is essentially still negligible.
This eliminates a number of problems arising from the fact that with
increasing expansion of the plasma, the plasma frequency and hence the
conditions for penetration of the light wave and the conditions for
optimum heating-up of the plasma by the laser beam also change.
A particularly preferred quantitative relation for the pulse intensity
I.sub.L and the pulse duration .tau. results from the relation
I.sub.L .times..tau..apprxeq.H
with H lying between approximately 1 and approximately 50
watt-sec/cm.sup.2.
Further preferred quantitative values for the pulse duration are obtained
by these being selected with a wavelength of 0.2 .mu.m in the range of
between approximately 1 and approximately 10 psec.
Quantitative values can also be specified for the wavelengths of the laser
beam, it being preferable to work with wavelengths of the laser beam of
<0.3 .mu.m.
Preferred values for the mean kinetic energy of the ions and atoms in the
stream of particles are of the order of magnitude of approximately 10 eV
to approximately 100 eV.
In all of the embodiments of the inventive process described so far, no
further details were given about the laser that is used. Use of a
neodymium solid-state laser--designed as Nd-yag laser--has proven
particularly expedient. Such lasers have wavelengths of approximately 1
.mu.m.
As an alternative to this, it is, however, also advantageous to use a
Ti-sapphire laser. This laser has wavelengths of approximately 0.8 .mu.m.
Finally, it is also conceivable within the scope of an advantageous
solution of the inventive process to use an excimer laser.
To attain the short pulse durations according to the invention, it is
preferable for the laser radiation for generation of the pulse duration
according to the invention to be mode-coupled.
In addition, the inventive wavelength of the laser beam is advantageously
achieved by carrying out frequency multiplication of a fundamental
wavelength.
Further features and advantages of the invention are set forth in the
following description of an embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings show:
FIG. 1--an epitaxy system, operated in accordance with an embodiment of the
inventive process;
FIG. 2--a section taken along line 2--2 in FIG. 1; and
FIG. 3--an enlarged illustration of the front end of one of the pins
illustrated in FIG. 1.
DETAILED DESCRIPTION
An embodiment of the inventive process will be described with reference to
an epitaxy system with a mode-coupled and frequency-multiplied
titanium-sapphire laser illustrated in FIG. 1.
An ultra-high vacuum chamber 10 in which a substrate 12, for example,
silicon is arranged opposite an ablation area 14 is provided for carrying
out the process.
Pins 20 and 22 carried by a holder 16 and 18, respectively, and adapted to
be acted upon by a laser beam 28 and 30, respectively, at a front end 24
and 26, respectively, preferably at an end face, are arranged in this
ablation area 14. The pins 20 and 22, respectively, as carriers for the
coating material are themselves preferably made of the coating material.
The laser beams 28, 30 are focused on a spatially fixed point in the
ultra-high vacuum chamber 10 and so during the ablation from one of the
pins 20, 22, the respective pin has to be made to follow in the direction
of the focus of the respective laser beam 28, 30. This is preferably
carried out via the holder 16, 18, each of the holders being arranged in a
follow-up device 32 and 34, respectively, arranged outside the ultra-high
vacuum chamber 10 and being guided via a passageway 36 and 38,
respectively, into the ultra-high vacuum chamber 10 and slidable parallel
to a longitudinal axis 40 and 42, respectively, of the pins 20, 22 in the
ultra-high vacuum chamber 10. Hence continuous feeding of the pins 20 and
22, respectively, is carried out by this follow-up device 32 and 34,
respectively, so the end 24 and 26, respectively, of the respective pin 20
and 22, respectively, is acted upon by the focus of the respective laser
beam 28 and 30, respectively.
It is preferable for not only one pin 20 and 22, respectively, to be held
on each of the holders 16 and 18, respectively, but, as illustrated in
FIG. 2, for a number of pins 20a to h and 22a to h to be placed in a
turret arrangement, with one of the pins 20a to h and one of the pins 22a
to h being respectively positionable in the ablation area 14. In order to
subsequently position the pins 20a to h and 22a to h in the ablation area
14, each holder 16 and 18, respectively, is additionally rotatable about a
longitudinal axis 44 and 46, respectively, parallel to the longitudinal
axis 40 and 42, respectively, of the pins 20 and 22, respectively, with
rotation of the holders 16 and 18, respectively, likewise being carried
out via the respective follow-up device 32 and 34, respectively.
During the ablation, a stream of particles 48 and 50, respectively,
emanates from each of the pins 20 and 22, respectively, standing in the
ablation area 14, and the two streams of particles 48 and 50,
respectively, have such a beam angle that they coat the substrate 12 over
the entire desired surface.
The direction of the laser beams 28 and 30, respectively, is preferably
selected such that the streams of particles 48 and 50, respectively
propagate in a direction of propagation 52 which preferably stands
perpendicular on a surface 54 of the substrate 12. In the cases where the
beam angle of the streams of particles 48 and 50, respectively, is smaller
than the area of the substrate 12 to be coated, a sliding device 56 can be
additionally provided to slide the substrate 12 preferably perpendicular
to the direction of propagation 52. Insofar as the area acted upon by the
respective stream of particles 48 and 50, respectively, i.e., the
cross-sectional area of the respective stream of particles 48 and 50,
respectively, is to be additionally varied, the sliding device 56 can,
however, also serve to slide the substrate 12 in the direction of
propagation 52.
The two laser beams 28 and 30, respectively, are generated by a
titanium-sapphire laser designated in its entirety 60 and comprising a
resonator which has two end mirrors 62 and 64 and in the beam path 66 of
which a titanium-sapphire crystal 68 is arranged. Also arranged in the
beam path 66 are a mode locker 70 and a Pockels cell 72, with the latter
serving to turn the polarization in the beam path 66. A selection prism 74
is provided for wave selection in the beam path 66 and coupling-out is
carried out via a Brewster reflector which allows a coupled-out beam 78 to
exit transversely to the beam path 66. This coupled-out beam 78 strikes a
deflection mirror 80 which first sends the coupled-out beam through a
titanium-sapphire amplifier 82 to further amplify the power of the
coupled-out beam 78.
A reinforced laser beam 84 emerging from the titanium-sapphire amplifier
with a wavelength of 800 nm is doubled in a frequency doubler 86 and
continues as doubler laser beam 88 and is doubled again in a further
frequency doubler 90, which finally results in an output laser beam 92
with a wavelength of 200 nm.
This output laser beam 92 strikes a rotating mirror 94 which is fixable in
two positions. In a first position, illustrated in continuous lines in
FIG. 1, the output laser beam 92 is reflected onto a further deflection
mirror 96 and passes from the latter as laser beam 28 through an optical
focusing device 98 into the ultra-high vacuum chamber 10.
Alternatively to the first position, the rotating mirror 94 can be brought
into a second position, illustrated in dashed lines in FIG. 1, so the
output laser beam 92 is directly reflected from the rotating mirror 94 via
an optical focusing device 100 into the ultra-high vacuum chamber as laser
beam 30. The optical focusing devices 98 and 100, respectively, are
metered such that the laser beams 28 and 30, respectively, are focused in
the intended ablation area 14, for example, such that they act on the
entire end face of the respective pin 20 and 22, respectively, standing in
the ablation position essentially over the entire area thereof and hence
bring about ablation on the entire surface.
The inventive epitaxy system according to FIG. 1 operates in such a way
that coating material is ablated either with laser beam 28 from pin 20 or
with laser beam 30 from pin 22. This makes it possible, for example, for
different coating materials to be subsequently deposited by evaporation in
different layers on the substrate 12 and for the desired layer structure
to be thereby obtained. In addition, the arrangement of several pins 20a
to h and 22a to h, respectively, offers the possibility of also depositing
more than two coating materials by different pins 20a to h and 22a to h,
respectively, being made of different coating materials. Hence changeover
from pin 20a to pin 20c and back is, for example, also possible.
The relations during the ablation are illustrated by way of example on a
non-uniformly enlarged scale in FIG. 3 with ablation from pin 20 by means
of laser beam 28. The laser beam 28 is focused on an end face 102 of the
pin 20a such that as focusing area 104 covers the entire end face 102 and
generates in a disc having a thickness d a plasma which, in accordance
with the invention, represents a cluster-free plasma containing single,
non-coherent atoms or ions.
The conditions for the intensity I required in accordance with the
invention result from the assumption that the pulse duration .tau..sub.O
is to be so short that there will be no expansion of the plasma generated
in the layer of thickness d during the pulse duration .tau..sub.O. In this
case, the intensity I.sub.O is approximately
I.sub.O .times..tau..sub.O =d.times.N.sub.F .times.E.sub.i
with N.sub.F as particle density in the solid state and E.sub.i as kinetic
energy of the ions or atoms in the plasma, with the ionization energy
being neglected.
Furthermore, by placing
##EQU16##
with v.sub.i as velocity of the ions or atoms in the expanding plasma, one
obtains an approximate formula
##EQU17##
with M as mass of the ions or atoms in the plasma.
On the basis of an approximate, one-dimensional expansion of the plasma,
one can apply the relation for the intensity I.sub.L for longer pulse
durations
##EQU18##
with v.sub.i .times..tau. as thickness of the layer of the expanding
plasma and N.sub.P as particle density in the expanding plasma.
Furthermore, the known relation for the plasma frequency
##EQU19##
with m as electron mass leads to the relation
##EQU20##
if one further assumes that the angular frequency .omega..sub.L
corresponding to the wavelength of the laser beam is to be identical with
the plasma frequency .omega..sub.P of the plasma.
This assumption results from the fact that penetration of the laser beam
and hence intensive interaction with the plasma are only possible when
.omega..sub.L is >.omega..sub.P, whereas when .omega..sub.L is
<.omega..sub.P the plasma reflects the laser beam and, therefore, also the
conditions for heating-up and ionization are unfavorable.
With these considerations, a relation for the pulse duration .tau. can also
be set up as follows:
##EQU21##
with
.tau..sub.O =d.times.E.sub.i.sup.-1/2 .times.M.sup.1/2
This is achieved by way of example by the focusing area having a diameter
of 0.2 mm and being irradiated with 10.sup.12 W/cm.sup.2. In this case,
the laser power is approximately 3.10.sup.8 W, the pulse duration
approximately 10 psec, the pulse energy 3 mWsec, the wavelength 200 nm and
the repetition frequency of the individual pulses approximately 100 Hz.
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